1. Field of the Invention
The present invention relates to an exhaust gas recirculation (EGR) valve provided in an EGR passage of an EGR apparatus and to be driven by an actuator.
2. Related Art
As the above type of technique, there is conventionally known an exhaust gas recirculation (EGR) valve 61 of a poppet valve structure as shown in FIGS. 7 and 8, for example. This conventional EGR valve 61 is provided with a housing 62 including a passage 63 for EGR gas, a valve seat 64 provided at some place in the passage 63, a valve element 65 provided to be seatable on the valve seat 64 and to form a measuring section 80 for EGR gas between the valve element 65 and the valve seat 64, a valve stem 66 provided, at its one end, with the valve element 65 to move the valve element 65 with respect to the valve seat 64, and an actuator 68 to make stroke movement of the valve stem 66 together with the valve element 65 in an axial direction. The valve stem 66 is stroke-moved by the actuator 68 to adjust an opening degree of the measuring section 80 and regulate a flow rate of EGR gas allowed to flow through the passage 63. Accordingly, gas flow rate characteristics of the conventional EGR valve 61 is defined by the shapes of the valve element 65 and the valve seat 64. JP-A-2008-202516 discloses one example of this type of EGR valve. FIG. 7 is a cross sectional view showing a fully closed state of the conventional EGR valve 61. FIG. 8 is a cross sectional view showing a fully open state of the conventional EGR valve 61.
Meanwhile, the present applicant analyzed a flow of EGR gas in the conventional EGR valve 61 and found that a flow of EGR gas is blocked or disturbed due to the shape of the measuring section 80 defined by the valve seat 64 and the valve element 65. Specifically, in the conventional EGR valve 61, the valve seat 64 is annular and has a valve hole 64a at the center thereof. The valve element 65 has a nearly flattened conical shape. In FIGS. 7 and 8, the valve seat 64 includes a first end face 64b and a second end face 64c arranged in an axial direction. The inner peripheral surface of the valve hole 64a is tapered so that a half of the valve hole 64a in the axial direction has an inner diameter increasing toward a downstream side of EGR gas, that is, toward the first end face 64b. The valve element 65 is configured such that, in a fully closed state as shown in FIG. 7, a minimum outer-diameter portion 65a of the valve element 65 is placed inside the valve hole 64a and a portion around a maximum outer-diameter portion 65b of the valve element 65 is in contact with an inner peripheral edge of the second end face 64c of the valve seat 64 to close the valve hole 64a. 
FIG. 9 is an explanatory diagram showing CAE analysis result of an EGR gas flow (EGR gas flow direction and EGR gas flow velocity distribution) in the passage 63 in the fully open state. FIG. 10 is an explanatory diagram similarly showing CAE analysis result of the EGR gas flow (EGR gas flow velocity distribution) in the passage 63 in the fully open state in the conventional EGR valve 61. In FIGS. 9 and 10, the EGR gas flow along the inner peripheral surface of the passage 63 impinges on the second end face 64c of the valve seat 64 and changes the orientation by 90° toward the valve hole 64a. Thereafter, the EGR gas flow passes through the measuring section 80 defined between the valve seat 64 and the valve element 65 and goes toward the downstream side of the passage 63 while spreading along the tapered shape of the valve hole 64a. As shown in FIGS. 8 and 9, in the conventional EGR valve 61, at the second end face 64c of the valve seat 64, the opening area of the measuring section 80 is minimum. Accordingly, the EGR gas flow from upstream of the measuring section 80 is constricted by the measuring section 80, and the flow toward downstream of the measuring section 80 spreads from the measuring section 80, resulting in large pressure loss. Thus, before and behind the measuring section 80, stagnation and peel-off (eddy) of the EGR gas flow are caused, leading to blockage or disturbance of the EGR gas flow. As shown in FIGS. 9 and 10, the EGR gas flow spreads when passing through the valve seat 64, so that the flow velocity is relatively increased in a position close to the center (valve stem 66) of the passage 63, but the flow velocity is decreased under the influence of eddy flow and others in a position close to the outer circumferential side in the passage 63. The flow velocity of EGR gas is relatively increased only in an area near the valve stem 66 and the flow velocity is not so increased in other areas.
Herein, for example, JP-A-9(1997)-42072 discloses an EGR valve configured such that the shape of a valve seat is designed to accurately control an EGR gas flow rate in a low opening region and ensure a maximum flow rate of EGR gas in a high opening region. As shown in FIG. 11, a housing 82 of this EGR valve is provided, in a portion corresponding to a valve seat, with a diameter increasing portion 87 that gradually widens a gas passage from a seating portion 84 on which a plate-like valve element 85 is seatable, toward a valve opening side. This diameter increasing portion 87 has a convex-curved shape having an inner diameter gradually increasing toward an upstream side. The diameter increasing rate of a zone 88 of the diameter increasing portion 87 on the valve opening side is set to be larger than the diameter increasing rate of a zone 89 on the side close to the valve seating portion 84. FIG. 11 is an enlarged cross sectional view showing part of the housing of the EGR valve.